In recent years, there has been progress in the commercialization of exposure apparatus that perform exposure by projection, onto a photosensitive substrate, of a mask pattern using extreme ultraviolet (EUV) light in the wavelength range of 5-40 nm as exposure light. In an EUVL (extreme ultraviolet lithography) exposure apparatus, optical materials are limited to materials that exhibit high transmittance at short wavelengths. Consequently, reflective optical systems are used (see, for example, U.S. Pat. No. 6,452,661, corresponding to Japan Laid-open (Kôkai) Patent Document No. Hei 11-312638, incorporated herein by reference).
An exposure apparatus as noted above is exemplified by the prior-art system shown in FIG. 9. A target material, supplied by a nozzle 203, is irradiated at a point 204 with laser light emitted from a non-EUV laser-light source 201 and condensed by a condensing element 202. Thus, the target material receives an intensely concentrated dose of radiant energy that converts the target material into a plasma that generates EUV light. The generated EUV light is condensed by a condensing mirror 205 and reflected by a concave mirror 206. The EUV light then is incident on an incidence-side fly-eye mirror 207, in which numerous concave mirror segments are arranged in rows. The beam reflected by the incidence-side fly-eye mirror 207 passes through a diaphragm aperture 208 and is reflected by an emission-side fly-eye mirror 209, in which numerous concave mirror segments are arranged in rows. The beam again passes through the diaphragm aperture 208 and is incident on an optical assembly 210. The beam reflected by the optical assembly 210 is condensed by a condensing optical assembly 211 for irradiation of a pattern-defining mask 212. The components and systems situated between the point 204 and the mask 212 constitute an “illumination system.” The beam carries an image of the pattern defined on the irradiated mask 212 as the beam passes through a projection-optical system 213, which projects the image onto a wafer (photosensitive substrate) 214. Thus, the wafer 214 is exposed with the pattern.
In an illumination system of a conventional exposure apparatus as summarized above, the incidence-side fly-eye mirror 207 and the emission-side fly-eye mirror 209 are separated from each other by a short distance. Consequently, the incidence angle of the beam on the incidence-side fly-eye mirror 207 is large. In an EUVL exposure apparatus, the reflective surfaces of the optical elements typically each include a surficial multilayer film that confers high reflectivity of the surface to incident EUV light. Maximal reflectivity of a multilayer film to EUV light is obtained whenever the EUV light is incident to the multilayer film at a small angle of incidence, i.e., nearly perpendicular to the film.
In the system shown in FIG. 9, to reduce the angle of incidence of EUV light on the incidence-side fly-eye mirror 207, the distance between the incidence-side fly-eye mirror 207 and the emission-side fly-eye mirror 209 must be increased. To accommodate this greater distance, the focal length of the condensing optical assembly 211 would have to be increased, which would result in the condensing optical assembly 211 being situated below the level of the wafer 214 rather than within the projection-optical system 213 as shown in FIG. 9. Because the wafer 214 moves in a scanning direction during exposure, placing the condensing optical assembly 211 below the level of the wafer 214 would cause the wafer 214 to interfere physically with the condensing optical assembly 211. Thus, in a conventional exposure apparatus as shown, it is difficult to satisfy both requirements of: (a) the condensing optical assembly 211 being provided with a long focal length to improve the reflectivity of the fly-eye mirrors, and (b) the distance between the incidence-side fly-eye mirror 207 and the emission-side fly-eye mirror 209 being increased so that EUV light is incident at an angle close to perpendicular on the reflecting surfaces of the incidence-side fly-eye mirror 207.